RadioBanter - View Single Post

Owen Duffy · January 9th 12, 04:21 AM posted to rec.radio.amateur.antenna

"Wayne" wrote in
:

"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
.
..

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

At any rate, I have bought enough telescoping tubing sections to build
about a 20 foot vertical. Final height is subject to approval by "the
Admiral". Fabrication of a more substantial base support will be
necessary.

In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet
of telescoping tubing, using the same mobile whip mount clamped to
conduit. Resonance is around
18.7 MHz and it loads up nicely on 18 through 10, with 10 meters
having the highest calculated swr of 17.

With the antenna removed, the feedline shows a 1/4 wave null at
4.33MHz. This gives an estimated 1/2 wave electrical length of 57 feet
at 8.66 MHz, or with 0.66 VF a physical length of 37.6 ft. That seems
about right.

I'll get a Smith chart and see where the existing feedline moves the
parameters. Per your comments I'll try to avoid low r values.

Wish the bands would open back up like they were in Oct and Nov.
-
Wayne
W5GIE

So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen